1. Field of the Invention
The present invention relates generally to audio transducers and specifically with the compensation of transducer characteristics.
2. Related Art
An audio transducer converts between electrical energy and sound energy. Well known for the conversion from electrical to sound are speakers and headphones and from sound to electrical are microphones. Unfortunately, audio transducers inherently do not accurately reproduce the signal presented to them at the input. For example, the frequency response across the entire audible spectrum is seldom flat. Meaning in the case of reproduced sound, the speakers or headphones would reproduce some frequencies more loudly than others.
Traditionally, equalizers have been used to compensate for the inaccuracies in reproduction and/or recording. Even in old stereo equipment, graphic equalizers were available to correct for spectral variations in the sound reproduction. The difficulty in this approach is that the user relies on hearing and personal preferences to manually manipulate a series of filters. The number of controls is small and the adjustment could only be made very coarsely.
FIG. 1 shows a sound recording and/or reproduction system typical of modern personal computers (PC) and personal electronic devices. In the reproduction path of host device 100, a digital electrical sound signal is received from source 102. Examples of source 102 can be storage 104 or alternate source 106 which can be a computer network As an example, storage 104 can contain a song which is stored on hard disk. An example of alternate source 106 is a streaming source, such as a live radio broadcast over the Internet. Often the alternate source 106 uses storage 104 to buffer the signal. The digital electrical sound signal is processed by digital stage 108 which often comprises digital signal processor (DSP) 110. DSP 110 may be coupled to memory 112 and/or central processing unit (CPU) 114. Often a single processor functions as both DSP 110 and CPU 114 especially for personal electronic devices. Memory 112 and/or CPU 114 can control the retrieval of the digital electrical sound signal as well as direct the DSP as to the type of processing to be performed. The processed digital electrical sound signal is converted to an analog electrical sound signal by digital to analog converter (DAC) 116. The analog electrical sound signal is then processed by analog stage 118 which often comprises amplifier 120. The analog electrical sound signal then drives a transducer such as headphone 122 or speaker 124 which produces an acoustical sound signal.
On the recording path, a transducer such as microphone 152 records an acoustical sound signal into an analog electrical sound signal, which is processed by analog stage 118 which often comprises amplifier 154. Analog stage 118 conditions the analog electrical sound signal so it can be converted to digital by analog to digital converter (ADC) 156. The digital electrical sound signal is then processed by digital stage 108 which often comprises DSP 110. Like in the reproduction path, DSP 110 can at the direction memory 112 and/or CPU 114 further process the digital electrical sound signal. The signal can then transmitted to receiving medium 158. For example, the signal can be stored as a sound file in 104 or transmitted over computer network 160.
DSP 110 is capable of compensating for the inaccuracies in the reproduction of sound due to the characteristics of the transducers. If the characteristics of the transducers are known, the electrical audio signal can be preconditioned to compensate for the inaccuracies of the audio transducers before reproducing the sound, and similarly the inaccuracies of an audio transducer can be corrected for recording a sound.
Once the characteristics of a transducer are known, many additional DSP algorithms can be applied in order to improve the audio performance and even safety of the system. As a simple example, the transfer function of a speaker can be measured. The speaker inaccuracies present in the transfer function can be compensated for by applying a filter with the inverse of the transfer function to the electrical audio signal prior to supplying the signal to the speaker. The net result is that the effect of the filter and the inaccuracies of the speaker cancel out.
In the past, even before the ubiquity of DSPs, some high end speaker manufacturers provided a specific equalizer to compensate for the known frequency response deficiencies of their speakers. In the PC or personal electronic devices, the characteristics of internal speakers as well as built-in microphones can be stored on a hard drive, read-only memory (ROM) or some other form of non-volatile memory. The onboard DSP can read this data and make appropriate compensation.
However for external speakers, headphones or microphones, the precise transducer characteristics are not known. Because the transducer characteristics can vary greatly, no universal compensation technique could be applied with much success.
At present, the best PC's and other personal electronics offer is to present a graphic equalizer that enable the user to adjust the frequency response by hand. FIG. 2 shows the same audio processing system as shown in FIG. 1. The diagram is expanded to show display 202 which for a computer can be a monitor and user input-output (I/O) device 204, shown here for a computer as keyboard and mouse, but for personal electronics devices could be a touchpad, wheel, or a variety of other interfaces. Shown on display 206 is a virtualization of a graphic equalizer, the user can use I/O device 204 to manipulate sliders within the graphic equalizer to adjust the gain or attenuation over a certain frequency band, emulating the graphic equalizer seen in many stereo systems. Alternatively a limited set of equalization presets are made available for the user to select from. Neither approach can offer the granularity needed for proper compensation.
Proper compensation generally needs parametric equalization where each filter's center frequency, amplitude and bandwidth can be adjusted. While parametric equalization could be made available to the end user. Short of being a sound engineer, a user would find these parameters too complicated to comprehend let alone adjust. Furthermore, the typical end user does not have the proper equipment to measure the frequency response. In an ideal setting, the right bandwidth, gain and center frequency of a parametric equalizer would require calibrated microphones. In the case of headphones, a “head and torso simulator” device is required to mimic the response of a headphone when sealing to an actual human head.
In addition to accurate reproduction, speakers and headphones may be further constrained. For safety, the headphone output should be limited to a certain sound pressure level (SPL). Many safety mechanisms require the user to “guess” how loud his music is playing at as a result, the user can not really be assured that he is not damaging his hearing. To this end, the European union issues a Geprüfte Sicherheit (Tested Safety) Mark or GS Mark for portable media devices that limits output a certain number of millivolts. However, different transducers can be louder or quieter even at the same voltage output based on sensitivity and impedance. Therefore headphones with the GS-Mark standard can over-protect or under-protect based on its transducer.